SERVICE CONTINUITY FOR TODAY’S VOICE OVER LTE SUBSCRIBERS

7/23/2019 SERVICE CONTINUITY FOR TODAYS VOICE OVER LTE SUBSCRIBERS

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SERVICE CONTINUITYFOR TODAYS VOICEOVER LTE SUBSCRIBERSENSURING VOICE SERVICEWITH SINGLE RADIO VOICECALL CONTINUITY SRVCCTECHNOLOGY WHITE PAPER

Subscribers expectations for mobile data services are continuing to increase.

Mobile operators are responding to this demand by deploying 4G Long Term

Evolution (4G LTE) networks, which provide a better mobile data service at a

more attractive cost. Based on the initial commercial success of 4G LTE, mobile

operators are preparing to launch Voice over LTE (VoLTE), which unlocks all-IP

communications in the 4G LTE network. However, current 4G LTE service is not

as geographically widespread as 2G/3G, and subscribers are demanding the

confidence that their voice calls will not be dropped when they move out of

4G LTE coverage.

Mobile operators are therefore relying on a VoLTE technology called SingleRadio Voice Call Continuity (SR-VCC). With safe, rapid launches enabled by

SR-VCC, service providers can secure subscribers confidence in their voice

services while simultaneously unlocking 4G LTEs all-IP communications with

enriched voice, many forms of video, IP messaging and social communications.


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TABLE OF CONTENTS

1. Introduction / 1

2. Ensuring voice call service continuity / 2

2.1 SR-VCC definition and overview / 2

2.2 Voice Call Continuity / 4

2.3 VCC during emergency calls / 7

3. Ensuring VCC when roaming between networks / 10

3.1 Challenges for subscribers who roam internationally / 10

3.2 eSR-VCC definition and overview / 11

3.3 Enhanced VCC / 12

4. Conclusion / 12

5. Acronyms / 13

6. References / 14


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Service Continuity for Todays Voice over LTE Subscribers

ALCATELLUCENT WHITE PAPER

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1. INTRODUCTIONThe sudden and spectacular popularity of 3G data services observed in recent years

is rapidly overloading current 3G networks. Operators need to find new ways to

simultaneously boost the capacity of their mobile networks while optimizing operating

costs. Third-Generation Partnership Project (3GPP) 4G LTE is the perfect mix of

technological innovations that solve these needs. Today, 4G LTE is recognized as the

technology of choice for wireless network evolution. As of March 2012, 57 operators

have launched 4G LTE networks in 32 countries, and a further 59 operators in

14 additional countries are engaged in trials, tests or studies. [12]

4G LTE provides more bandwidth, less latency and Quality of Service (QoS) for the

delivery of data-intensive applications, such as mobile TV, over a simple, flat IP

architecture. 4G LTE offers mobile service providers an avenue to profitably deliver

next-generation wireless broadband services. Everyone agrees that 4G LTE is the way

to go for data. But how is voice service delivered to 4G LTE subscribers?

3GPP has standardized two methods for providing voice services in 4G LTE:

Circuit Switched Fallback (CSFB), in which voice calls are provided by the 2G/3G

Circuit Switched (CS) network. CSFB requires the User Equipment (UE, such as asmartphone) to fall back from 4G LTE to 2G/3G as soon as the subscriber originates

or receives a voice call (unless the subscriber rejects the incoming voice call).

VoLTE [10], in which voice calls are provided by the all-IP 4G LTE network,

including IP Multimedia Subsystem (IMS).

This document examines the VoLTE technology used to ensure that a stable voice call

is preserved when a subscriber moves out of 4G LTE coverage. This loss of 4G LTE

coverage may occur in:

4G LTE networks whose coverage is not yet as geographically widespread as that

of 2G/3G. For example, a subscriber moves out of 4G LTE coverage while traveling.

4G LTE networks that operate at very high frequencies, such as 2.6 GHz, with

line-of-sight transmission characteristics. For example, a subscriber moves out

of 4G LTE coverage when moving, even on foot or in a room.

In either situation the subscriber could lose their 4G LTE signal during a voice call. The

voice call is dropped unless it is gracefully handed over from 4G LTE to 2G Global System

for Mobile Communications (GSM) or 3G Universal Mobile Telecommunications System

(UMTS), also referred to as 3G Wideband Code Division Multiple Access (W-CDMA). Such a

graceful handover gives subscribers confidence in their voice service, and it distinguishes

the mobile operators VoLTE service from over-the-top (OTT) providers.

The VoLTE technology for this graceful handover is SR-VCC. A version called enhancedSR-VCC (eSR-VCC) applies for international roaming. Both are standardized by the 3GPP

as the means to ensure Voice Call Continuity (VCC) when a subscriber moves out of 4G

LTE coverage to 2G/3G CS voice coverage. Both SR-VCC and eSR-VCC ensure that the

handovers interruption of the voice call is less than 300 ms. The interruption occurs

when the UE transfers from 4G LTE to 2G/3G CS voice and the IMS reconnects from

the Evolved Packet Core (EPC) to the 2G/3G CS voice core. By keeping the voice

interruption to less than 300 ms, the effect on the subscriber is minimized. [2]


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The next update of the 3GPP standards (3GPP Release 11) will standardize the reverse

handover from 2G/3G CS voice to 4G LTE VoLTE. This is called Reverse SR-VCC (rSR-

VCC) and is not covered in this document. The value of rSR-VCC is to provide 4G LTE

data service for a subscriber who enters 4G LTE coverage during a 2G/3G CS voice call

(not to preserve a stable voice call because 2G/3G service is far more geographically

widespread than 4G LTE).

This document does not address voice service consistency across 4G LTE, 2G and 3Gnetworks. The value of voice service consistency is to ensure that a subscribers voice

services work similarly across all three mobile access networks. [9]

Section 2 describes SR-VCC, including principles relative to basic and emergency

calls, and the network implementation. Section 3 describes eSR-VCC, including why

an enhanced version is necessary, principles relative to basic calls and the network

implementation.

2. ENSURING VOICE CALL SERVICE CONTINUITY

In this section we describe the SR-VCC procedure, examine the functions that take placein the mobile network and IMS domain, address the network implementation and discuss

how emergency calls such as a subscribers call for help to police, fire or ambulance

are assured if the subscriber moves out of 4G LTE coverage during the call.

2.1 SR-VCC definition and overview

The 3GPP standard for SR-VCC, TS 23.216 [3], provides the following definition of

Single Radio Voice Call Continuity (SR-VCC): Voice call continuity between IMS over

Packet Switched (PS) access and Circuit Switched (CS) access for calls that are anchored

in IMS when the UE is capable of transmitting/receiving on only one of those access

networks at a given time.

There are two interesting points in this definition:

Single radio: The UE is in either 4G LTE or 2G/3G coverage but it cannot connect or

listen to the two types of radio access networks at the same time. Therefore, when the

UE moves from 4G LTE to 2G/3G, the UE is disconnected from 4G LTE and reconnects

to 2G or 3G. When this happens during a voice call, SR-VCC defines the procedures.

Outside of a voice call, SR-VCC does not apply: instead, cell reselection procedures are

used.[7]

The call must be anchored in the IMS domain: The voice call is controlled by the IMS

in the subscribers home network. To provide this control, both incoming and outgoing

calls are anchored in IMS. The call may be Voice over IP (VoIP) or CS voice.


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SR-VCC is defined by 3GPP standards TS 23.216, TS 23.237 [4] and TS 24.237 [6]. We

first describe the high-level SR-VCC procedure and then examine the mobile and IMS

components in Section 2.2.

As shown in Figure 1, the high-level SR-VCC procedure is:

1. Green line: The subscriber is in the midst of a stable VoLTE call.

2. Dashed purple line: The subscriber moves out of 4G LTE coverage, triggering the

SR-VCC mechanism. The EPC communicates with the specific Mobile SwitchingCenter (MSC) in the CS core that will receive the handover. The MSC triggers the

handover of the voice call from the 4G LTE radio to the 2G/3G radio. At the same time

the MSC triggers the IMS to perform a session transfer, establishing a new access path

with the MSC.

3. Solid purple line: The voice call is reestablished over the 2G/3G radio and the CS core.

The voice call remains under control of the IMS.

Figure 1. General SR-VCC procedure

Subscriberssmartphone

IP multimediasubsystem

4G LTE radios

2G/3G radios2

2

1

3

Evolvedpacket core

Circuitswitched core

1. Subscriber is in a VoLTE call.2. Subscriber moves out of 4G LTE coverage, initiating an SR-VCC handover.3. Subscriber is now in a CS voice call.

The SR-VCC procedure relies on the following components:

UE, which must include an SR-VCC client

Mobile network, with:

Evolved UMTS Terrestrial Radio Access Network (eUTRAN) (radio part)

EPC, particularly the Mobility Management Entity (MME)

CS core (only a few MSCs need to be upgraded with new software to support

the SR-VCC procedure)

IMS domain, particularly the Service Centralization and Continuity Application

Server (SCC-AS) and the Home Subscriber Server (HSS)


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2.2 Voice Call Continuity

As previously described, when a subscriber moves out of 4G LTE coverage during a

VoLTE call, the SR-VCC procedure initiates a handover that resumes the voice call over

2G/3G CS access. Therefore, two handovers take place simultaneously: one in the mobile

network and one in the IMS network.

2.2.1 Handover on the mobile side

Figure 2 shows a high-level view of the EPC and CS core mechanisms involved when asubscriber moves from 4G LTE to 2G/3G CS access.

Figure 2. SR-VCC procedure on the mobile side


IP multimediasubsystem

4G LTE radios

2G/3G radios

1

5

2

4a

3

4b

EPCs MME

Sv

CS coresMSC-S and MGW

(with STN-SR)

(with STN-SR)

On the mobile side, handover of a call from 4G LTE to the 2G/3G CS core comprises the

following steps:

1. Based on reports sent by the UE to the Evolved Node B (eNodeB) on radio conditions,

the eNodeB decides that a handover must take place.

2. The eNodeB sends a request for handover to the MME, with an indication that it is an

SR-VCC handover.3. The MME triggers the SR-VCC procedure with the MSC server (which must have the

SR-VCC software). The Sv reference point uses the UEs Session Transfer Number

for SR-VCC (STN-SR) information to uniquely identify the particular subscriber. The

STN-SR is a unique identifier per subscriber and is stored in the HSS.

4. The MSC server then:

a. Initiates the session transfer procedure with IMS

b. Coordinates the IMS session transfer with the CS handover procedure to the target

2G/3G cell

5. The target 2G/3G radio sends a message to the MSC server to accept the handover.

The MSC server sends the PS-CS handover response to MME, including the necessary

CS handover command information for the UE to access the 2G/3G radios.

If the target MSC server that supports the specific 2G/3G radios the subscriber is moving

to is different from the MSC server enhanced with SR-VCC software, the SR-VCC MSC

simply uses normal MSC-to-MSC handover procedures to complete the transfer.


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The initial eNodeB handover message to the MME indicates whether the handover is

CS-only or if the handover is CS + PS. If the UE is engaged in a data session at the

time of the voice call handover, the PS data session is handled by the MME with the

PS bearer splitting function. Based on the handover message received from the eNodeB,

the MME starts the handover of the data session in parallel with the SR-VCC procedure.

The handover of data PS bearer(s) is done according to the inter Radio Access Transfer

procedure defined in 3GPP TS 23.401. [5] The MME coordinates the Forward Relocation

Response from the SR-VCC and the PS handover procedure from 4G LTE to 3G PS.

2.2.2 Handover on the IMS side

The SR-VCC feature is intrinsically defined with IMS anchoring, ensuring IMS control for

both originating and terminating voice calls:

Originating calls: VoLTE calls originating in 4G LTE are naturally handled by the IMS.

If the subscriber is originating a call over 2G/3G CS access, the 2G/3G MSC and Home

Location Register (HLR) handle the call as a 2G/3G CS voice call, and IMS anchoring is

not needed or used. For the related topic of consistent services in 2G/3G CS voice and

4G LTE VoLTE, see Service Consistency for Todays VoLTE Subscribers. [9]

Terminating calls: For VoLTE subscribers, the incoming voice call is always routed to

the IMS, where it is anchored. When a subscriber is in 4G LTE coverage, the VoLTE

call is naturally handled by the IMS. When a subscriber is in 2G/3G CS voice coverage,

IMS anchoring ensures that the IMS provides terminating features.

In the VoLTE network, the 3GPP-defined SCC-AS is always inserted in the path of an IMS

voice call. The SCC-AS is triggered in the subscribers home mobile network. It is the

first AS used on originating calls and the last AS used on terminating calls. Because of

this sequencing, when an SR-VCC handover from 4G LTE to 2G/3G CS voice is triggered,

the SCC-AS is informed and provides the IMS session transfer procedure. The SCC-AS

takes care of all impacts related to the UEs mobility so that other IMS elements in the

call are unaffected by SR-VCC.

Figure 3 shows the high-level view of IMS mechanism involved when the VoLTEsubscriber moves from 4G LTE coverage to 2G/3G CS voice access.

Figure 3. SR-VCC procedure on the IMS side


MGWMGCF

TASSCC-AS

CSCF

4G LTE radios

2G/3G radios

Signaling andvoice path

IMS


1 2


EPCs S-GWand P-GW


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On the IMS side, the procedure for handing over a VoLTE call from 4G LTE to the

2G/3G CS core comprises the following steps:

1. The subscriber moves out of 4G LTE coverage, and from the mobile side the MSC

server triggers the IMS to perform a session transfer. The MSC server passes the

STN-SR identification to the SCC-AS, uniquely identifying the UE that is moving

out of 4G LTE coverage.

2. The IMS SCC-AS triggers a session transfer from the EPC to the CS core. A new voice

path is established to the CS core media gateway (MGW), and the old voice path to theEPC is released. The MSC then coordinates handover of the UE to the target 2G/3G radio.

The SR-VCC handover procedure moves one stable voice call. If the VoLTE subscriber

has multiple voice sessions, only the active voice call is handed over. The other calls,

such as a call on hold, are released.

2.2.3 Implementing SR-VCC in the UE and the network

The use of an SR-VCC handover implies that the 3G UTRAN does not support VoIP.

Otherwise, a simple Inter-Radio Access Technology (IRAT) PS handover from 4G LTE

to 3G UTRAN would occur, and the voice handover would not need to rely on the CS

domain and the SR-VCC mechanism.

This section and Table 1 describe SR-VCCs network implementation.

UE

The subscribers device must support SR-VCC and indicate to the 4G LTE network,

at the time of radio attachment, that SR-VCC is to be used for handovers to the

2G/3G CS network.

eUTRAN

In addition to VoLTE optimizations [10], the eUTRAN receives the radio reports from the

UE and instructs the MME when a handover is needed.

HSS

The HSS provides the STN-SR, which uniquely identifies the subscribers UE. The

STN-SR is downloaded from the HSS to the MME when the UE attaches to the eUTRAN

radio. The HSS also informs the MME when the STN-SR is modified or removed.

The HSS also supports the Terminating Access Domain Selection (T-ADS) parameter,

which identifies the types of access supported by the UE. The T-ADS information takes

into account the access networks capabilities, UE capabilities, IMS registration status,

CS status, existing active sessions, user preferences, and operator policies such as access

network-specific voice domain preferences.

In addition, the HSS manages failure cases for example, when it does not receive theT-ADS from the MME and therefore cannot provide the T-ADS to an AS. In this case, the HSS

sends an indication to the SCC-AS that the IMS Voice over PS session support is unknown.

MME

The MME distinguishes voice from the data PS bearers, initiates the SR-VCC handover

procedure to the target MSC over the Sv interface and, if a data PS session is handed over

simultaneously with the voice session, coordinates the CS and PS handovers. With the

HSS, the MME also processes the T-ADS. The MME also builds the mobility tracking area

(TA) list to ensure the consistent support of VoLTE in the EPC and IMS.


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MSC

The MSC server must be upgraded with the Sv interface. The MSC coordinates the

SR-VCC handover and initiates the session transfer procedure. Upon successful handover,

the MSC server also registers the UE with the 2G/3G HLR.

Not all MSC servers located in the 4G LTE area need to be upgraded to support the

Sv interface. Only the MSC servers that receive the handover need be upgraded. If the

target MSC that supports the 2G/3G radios does not have the SR-VCC software, theSR-VCC MSC simply performs a standard MSC-to-MSC handover to conclude the

SR-VCC handover.

SCC-AS

If not already present, the SCC-AS is introduced in the IMS network. Some networks have

already deployed the SCC-AS to support IMS call anchoring services, such as those used

for single number services across fixed and mobile. The SCC-AS provides the SR-VCC

function, shielding other IMS elements from the procedure. The SCC-AS interworks

with the HSS for the T-ADS selection of access networks.

Table 1. SR-VCC network implementation

NODE SR-VCC CAPABILITY

UE Addition of SR-VCC and IMS clientsProvides SR-VCC capability information to the MME at the time of radio attachmentHandover between VoLTE and 2G/3G CS voice

eUTRAN Receives radio reports from the UESignals the need for a handover to the MME

HSS STN-SR provisioning and error handlingT-ADS processing query

MME Sv interfaceChecks IMS roaming agreement with the home mobile networkT-ADS processing query Builds TA list that matches VoIP support

MSC server Sv interfaceSR-VCC handover processingSession transfer initiationRequired only on a subset of MSCs

SCC-AS Session transfer controlT-ADS control

2.3 VCC during emergency calls

In addition to the general voice calls described in Section 2.2, mobile operators must

support subscribers emergency calls for help from police, fire and ambulance. Such calls

must be assured to work, even if a subscriber moves out of 4G LTE coverage during an

emergency call. This paper examines the VoLTE situation. Alternatively, an operator can

support LTE subscribers emergency calls using 2G/3G CS voice, but this situation is not

covered because it does not use SR-VCC.


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2.3.1 Emergency calling

In this section, we provide a high-level description of emergency calling and then

examine SR-VCC.

Emergency calls are handled differently from normal calls to reduce call setup time,

determine the subscribers location, identify the correct Public Safety Answering Point

(PSAP) and allow emergency calls even if the device is not provisioned for service. For

emergency call processing, two functions are added to the IMS: the Emergency CallSession Control Function (E-CSCF) and the Emergency Access Transfer Function (EATF).

In an emergency call, the E-CSCF triggers the EATF. The EATF handles aspects related to

mobility, which shields other IMS components.

These procedures apply to the home network service. When roaming, emergency calls

including the EATF are supported by the visited network.

As shown in Figure 4, a VoLTE subscribers emergency call comprises the following

standard steps:

1. The UE initiates the emergency call.

2. The eUTRAN sets up emergency service radio bearers.

3. The IMS Proxy CSCF (P-CSCF) recognizes the emergency call and routes it to the E-CSCF.

4. The E-CSCF triggers the EATF for mobility processing. Because of the similarity with

the SR-VCC SCC-AS, these two functions are conveniently combined into one network

element, as indicated by the rectangle surrounding them.

5. The E-CSCF queries the Location Routing Function (LRF) to identify the correct PSAP

for the mobile UEs location.

6. The E-CSCF routes the call to the PSAP using the MGCF and MGW.

Figure 4. SR-VCC and emergency calling


MGWMGCF

LRFEATF

P-CSCF,E-CSCF

4G LTE radios

IMS

Public SafetyAnswering Point(PSAP)

SCC-AS

1

2

3 4 5 6

EPCs S-GWand P-GW


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Focusing on SR-VCC, emergency calls must be supported even if the subscriber moves

out of 4G LTE coverage during the call. The process for this support is similar to that of

the SR-VCC procedure on the IMS side (see Figure 3, steps 1 and 2), except the SCC-AS

role in a normal SR-VCC handover is instead provided by the EATF (see Figure 5).

Figure 5. SR-VCC handover of an emergency call


MGWMGCF

LRFEATF

P-CSCF,E-CSCF

4G LTE radios


EPCs S-GWand P-GW


IMS


Public SafetyAnswering Point(PSAP)

1 2

2G/3G radios

The operators investment in emergency calling is therefore leveraged for the VoLTE

subscribers. For example, VoLTE subscribers receive faster call setup and routing tothe correct PSAP, as previously noted. Their calls are prioritized and preserved even

if a subscriber moves out of 4G LTE coverage during an emergency call a situation

that was not a concern for 2G/3G CS voices geographically widespread access but is a

concern for 4G LTEs initial deployments. Even after an SR-VCC call handover to 2G/3G

CS voice, such emergency features as PSAP auto callback (in case of a disconnection)

continue to function.


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3. ENSURING VCC WHENROAMING BETWEEN NETWORKSVoLTE roaming enables subscribers traveling to or from other networks to enjoy the

all-IP 4G LTE service even when they travel. Some operators plan to launch home

network VoLTE ahead of VoLTE roaming (in which case they use CSFB for 4G LTE

roaming) and some operators plan to launch at the same time as VoLTE roaming.

VoLTE roaming is expected in one to two years, so operators must consider theirroaming strategy. As previously described, an SR-VCC voice handover is composed

of a mobile part and an IMS part. As discussed next, an IMS enhancement ensures

that the international roaming VoLTE calls handover is also a brief duration.

3.1 Challenges for subscribers who roam internationally

When a subscriber roams from the home VoLTE network to a visited VoLTE network on

a different continent, the transport delay increases the duration of the voice interruption

during an SR-VCC handover. This is caused by the additional distance between the

visited VoLTE networks SR-VCC MSC server and the home IMS networks SCC-AS. If

both parties in a VoLTE call were to roam to visited VoLTE networks, the effects would

be additive. As noted in Section 1, the SR-VCC voice interruption must be kept to lessthan 300 ms.

Figure 6 shows a roaming example. The European VoLTE user is roaming abroad to the

United States (US) and calls a person in Australia. When the VoLTE subscriber who is

roaming in the US undergoes an SR-VCC handover, the voice interruption will be too

long. Note that a VoLTE calling partys VoIP bearer is not required to route through the

home network in the GSMAs CS Copycat roaming method. [10]

Figure 6. SR-VCC handover of an international roaming call

VoLTE UE

Other party

MSC-S

1 2

MME SCC-AS

MGW

Mobilehandover, brief

IMS sessiontransfer, long

VoIP bearerafter handover

CS voice bearerafter handover

VoIP bearerbefore handover

Visited VoLTE networkUS

Home VoLTE networkFrance

Other partyAustralia

MSC-S does in parallel:1. Handover command2. IMS session transfer


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3.2 eSR-VCC definition and overview

The 3GPP defined the eSR-VCC mechanism [1] [4] [6] to reduce the duration of the

voice interruption, thereby solving the international roaming problem described in

Section 3.1. Emergency calling is not a concern for eSR-VCC because emergency

services are provided by the visited network, not the home IMS network.

The duration of the voice interruption is caused by the lengthy IMS session transfer

shown in Figure 6. It is reduced by moving the IMS anchor point closer to the roamingVoLTE subscriber. This is accomplished by two functions: Access Transfer Control

Function (ATCF) and Access Transfer Gateway (ATGW), both shown in Figure 7.

The ATCF is inserted in the path at the time of the UEs radio attachment and IMS

registration. The ATCF sends an STN-SR address (pointing to the ATCF) to the MME.

During the SR-VCC procedure, when the MME sends the STN-SR to the MSC server

(see Section 2.2.1, Step 3), subsequent processing uses the ATCF and ATGW located

in the visited VoLTE network, not the home VoLTE network.

The call is therefore anchored in the visited VoLTE network. Regardless of the distance

between the two UEs and the distance between the VoLTE UE (which is moving out of

4G LTE coverage) and the home VoLTE network, the handover duration is always that

of a local, in-country handover.

Figure 7. eSR-VCC handover of an international roaming call

VoLTE UE

Other party

MSC-S

1 2

MME ATCF

ATGWMGW

Mobilehandover, brief

IMS sessiontransfer, brief

UnchangedVoIP bearer

VoIP bearerafter handover

CS voice bearerafter handover

VoIP bearerbefore handover

Visited VoLTE networkUS

Home VoLTE networkFrance

Other partyAustralia

MSC-S does in parallel:1. Handover command2. IMS session transfer


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3.3 Enhanced VCC

3.3.1 Handover and implementation on the mobile side

In the mobile network, eSR-VCC functions the same as SR-VCC, described in Section 2.1.

Both methods require the same network functions, described in Section 2.2.3. If SR-VCC

is already deployed into the mobile network, the eSR-VCC method is available.

3.3.2 Handover and implementation on the IMS side

As described in Section 3.2, the eSR-VCC mechanism introduces the ATCF and ATGW.Located in the visited VoLTE network, the ATCF anchors IMS signaling and the ATGW

anchors IMS media. Typically the ATCF is combined with the P-CSCF and the ATGW is

combined with the borders media element, providing better performance at a lower cost.

The ATCF is inserted in the IMS signaling path during the IMS registration procedure.

During IMS registration, the ATCF provides an STN-SR (pointing to itself) and forwards

it to the SCC-AS. The SCC-AS is enhanced to update the HSS with the ATCF STN-SR.

The HSS is enhanced to use this updated STN-SR and forward it to the MME. As noted in

Section 3.3.1, the MME is not affected by the eSR-VCC method, but it does use the ATCF

STN-SR when invoking subsequent voice call handover.

The visited networks ATCF and the home networks SCC-AS act in cooperation to

perform the actual eSR-VCC voice call handover.

4. CONCLUSIONOperators such as AT&T [11] are preparing for the imminent deployment of VoLTE.

Because most operators 4G LTE access does not initially extend to their entire 2G/3G

geographical coverage, the SR-VCC method provides essential VCC for subscribers who

move out of 4G LTE coverage during a voice call. For operators who expect international

VoLTE roaming, the eSR-VCC method ensures that their subscribers also enjoy minimal

interruptions in their voice calls.

VoLTE is a serious engineering undertaking, and service providers need a partner who

can help them follow a safe, profitable path. Four operators have contracted Alcatel-

Lucent for commercial service, and we have over 13 trials covering all regions of the

world. Alcatel-Lucent is implementing VoLTE, including SR-VCC and eSR-VCC, as part

of the Alcatel-Lucent 4G Consumer Communications solution [8]. This solution assures

subscribers of global roaming, service interoperability and performance.

Alcatel-Lucent implements the new functions in proven products, which enables

cost-effective deployment and improves reliability:

The SCC-AS of Figures 3, 4 and 6 and the emergency services EATF of Figures 4

and 5 is implemented in the Alcatel-Lucent 5420 Services Continuity Gateway,which is already commercially deployed for voice call continuity services.

The roaming services ATCF of Figure 7 is implemented in the widely deployed

Alcatel-Lucent 5450 IP Session Controller and the Alcatel-Lucent 5060 IP Border

Controller (both are co-located along with the P-CSCF), which ensures the roaming

subscribers QoS is extended from the home network to visited networks.

The roaming services ATGW of Figure 7 is implemented in the Alcatel-Lucent 7510

Media Gateway (MGW), which is widely deployed for IP border and IP-to-TDM media

gateway services.


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By accelerating VoLTEs launch with the performance of SR-VCC and eSR-VCC, operators

can use VoLTE to innovate and extend mobile voice beyond a traditional call. Voice can

become a feature of other services, such as navigation, e-commerce, social networking,

status updates and augmented reality applications.

With safe, rapid launches enabled by Alcatel-Lucents implementation of SR-VCC and

eSR-VCC, service providers can secure subscribers confidence in their voice services

while simultaneously unlocking 4G LTEs all-IP communications.

5. ACRONYMS2G Second-generation wireless, such as GSM

3G Third-generation wireless, such as UMTS/WCDMA

3GPP Third Generation Partnership Project

4G Fourth generation, such as LTE

ATCF Access Transfer Control Function

ATGW Access Transfer Gateway

CS Circuit Switched

CSCF Call Session Control FunctionCSFB Circuit Switched Fallback

E-CSCF Emergency CSCF

eUTRAN Evolved UMTS Terrestrial Radio Access Network

EATF Emergency Access Transfer Function

eNodeB Evolved Node B

EPC Evolved Packet Core

EPS Evolved Packet System

eSR-VCC Enhanced Single Radio Voice Call Continuity

GSM Global System for Mobile Communications

HLR Home Location Register

HSS Home Subscriber Server

IMS IP Multimedia Subsystem

IP Internet Protocol

IRAT Inter-Radio Access Technology

LRF Location Routing Function

LTE Long Term Evolution

MGCF Media Gateway Control Function

MGW Media Gateway

MME Mobility Management Entity

MSC Mobile Switching Center

MSC-S Mobile Switching Center- Server

OTT over-the-top

P-CSCF Proxy CSCF

PS Packet switched

PSAP Public Safety Answering Point

QoS Quality of Service

RoHC Robust Header Compression

rSR-VCC Reverse SR-VCC

SCC-AS Service Centralization and Continuity Application Server

SPS Semi-Persistent Scheduling

SR-VCC Single Radio Voice Call Continuity


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STN-SR Session Transfer Number for SR-VCC

T-ADS Terminating Access Domain Selection

TA tracking area

TAS Telephony Application Server

TTI Transmission Time Interval

UE User Equipment

UMTS Universal Mobile Telecommunications System

VoIP Voice over IP

VoLTE Voice over LTE

W-CDMA Wideband Code Division Multiple Access

6. REFERENCES

[1] 3GPP. TR 23.856, Single Radio Voice Call Continuity (SR-VCC) enhancements; Stage 2.

http://www.3gpp.org/ftp/Specs/html-info/23856.htm

[2] 3GPP. TS 22.278, Service requirements for the Evolved Packet System (EPS).


[3] 3GPP. TS 23.216, Single Radio Voice Call Continuity; Stage 2.

http://www.3gpp.org/ftp/Specs/html-info/23216.htm[4] 3GPP. TS 23.237, IP Multimedia Subsystem (IMS) Service Continuity; Stage 2.


[5] 3GPP. TS 23.401, General Packet Radio Service (GPRS) enhancements for Evolved

Universal Terrestrial Radio Access Network (eUTRAN) access.


[6] 3GPP, TS 24.237, IP Multimedia (IM) Core Network (CN) subsystem IP Multimedia

Subsystem (IMS) service continuity; Stage 3.


[7] 3GPP, TS 25.304, User Equipment (UE) procedures in idle mode and procedures for

cell reselection in connected mode.http://www.3gpp.org/ftp/Specs/html-info/25304.htm

[8] Alcatel-Lucent. 4G Consumer Communications solution, web page.

www.alcatel-lucent.com/4g-consumer-communications

[9] Alcatel-Lucent. Service Consistency for Todays VoLTE Subscribers. November 2011.


[10] Alcatel-Lucent. Voice over LTE: The New Mobile Voice. May 2012.


[11] Fierce Broadband Wireless. AT&Ts Rinne details LTE plans: VoLTE in 2013, web

site announcement. Oct. 20, 2010.

http://www.fiercebroadbandwireless.com/story/ts-rinne-details-lte-plans-volte-2013-

will-use-aws-and-700-mhz/2010-10-20

[12] GSM Suppliers Association. GSA Confirms Over 300 Operators Are Investing in LTE.

March 14, 2012. http://www.gsacom.com/news/gsa_349.php


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www.alcatel-lucent.com Alcatel, Lucent, Alcatel-Lucent and the Alcatel-Lucent logo are trademarks ofAlcatel-Lucent. All other trademarks are the property of their respective owners. The information presented

is subject to change without notice. Alcatel-Lucent assumes no responsibility for inaccuracies contained herein.

Copyright 2012 Alcatel-Lucent. All rights reserved. M2012042722 (April)

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SERVICE CONTINUITY FOR TODAY’S VOICE OVER LTE SUBSCRIBERS